Geographic data and visualization

GEOG 30323

November 17, 2015

What is geographic data?

  • Thus far: we’ve been working with data (largely) in tabular data frames, where each column represents data “attributes”
  • Geographic data: includes information on location - specifically, where the observation is located on the Earth’s surface

Maps!

Longitude and latitude: the basics

Geographic coordinates

  • Longitude (X) and latitude (Y) coordinates can be expressed as:
  • Degrees minutes seconds (e.g. \(97^{\circ}21'37''W\), \(32^{\circ}42'38''N\))
  • Decimal degrees (e.g. \(-97.36\), \(32.71\))

Conversion between DMS and decimal degrees:

\[DD = D + \frac{M}{60} + \frac{S}{3600}\]

Coordinate systems

  • Geographic coordinate system: coordinate system based on latitude and longitude (coordinates on a sphere)
  • Projected coordinate system: coordinate system projected onto a two-dimensional surface (coordinates on a plane)

Spheres and planes

Source: Mike Bostock

Map projections

Original source: Mike Bostock

Tiled mapping and Web Mercator

Map projections and distortion

Source: Curran Kelleher

Map projections and distortion

Source: Ian Johnson

Vector data

Raster data

Vector data: CSV files

Let’s make a map!

Mapping point data

Vector data: the shapefile

  • Shapefile: common format for encoding vector geographic data
  • Three required files: .shp, .dbf, and .shx; .prj recommended

Vector data: GeoJSON

Mapping polygons

Should you map?

  • Beware the “amazing map”…
Source: Clickhole (The Onion)

Maps vs. charts

  • For discussion: which visualization is more effective for showing differences in our data?

Chart for comparison